1 files changed, 54 insertions, 13 deletions
diff --git a/python/qemu/qmp.py b/python/qemu/qmp.py
index e64b6b5faa..7935dababb 100644
--- a/python/qemu/qmp.py
+++ b/python/qemu/qmp.py
@@ -12,13 +12,32 @@ import errno
 import socket
 import logging
 from typing import (
+    Any,
+    cast,
+    Dict,
     Optional,
     TextIO,
     Type,
+    Tuple,
+    Union,
 )
 from types import TracebackType
 
 
+# QMPMessage is a QMP Message of any kind.
+# e.g. {'yee': 'haw'}
+#
+# QMPReturnValue is the inner value of return values only.
+# {'return': {}} is the QMPMessage,
+# {} is the QMPReturnValue.
+QMPMessage = Dict[str, Any]
+QMPReturnValue = Dict[str, Any]
+
+InternetAddrT = Tuple[str, str]
+UnixAddrT = str
+SocketAddrT = Union[InternetAddrT, UnixAddrT]
+
+
 class QMPError(Exception):
     """
     QMP base exception
@@ -43,6 +62,25 @@ class QMPTimeoutError(QMPError):
     """
 
 
+class QMPProtocolError(QMPError):
+    """
+    QMP protocol error; unexpected response
+    """
+
+
+class QMPResponseError(QMPError):
+    """
+    Represents erroneous QMP monitor reply
+    """
+    def __init__(self, reply: QMPMessage):
+        try:
+            desc = reply['error']['desc']
+        except KeyError:
+            desc = reply
+        super().__init__(desc)
+        self.reply = reply
+
+
 class QEMUMonitorProtocol:
     """
     Provide an API to connect to QEMU via QEMU Monitor Protocol (QMP) and then
@@ -99,7 +137,10 @@ class QEMUMonitorProtocol:
             data = self.__sockfile.readline()
             if not data:
                 return None
-            resp = json.loads(data)
+            # By definition, any JSON received from QMP is a QMPMessage,
+            # and we are asserting only at static analysis time that it
+            # has a particular shape.
+            resp: QMPMessage = json.loads(data)
             if 'event' in resp:
                 self.logger.debug("<<< %s", resp)
                 self.__events.append(resp)
@@ -194,22 +235,18 @@ class QEMUMonitorProtocol:
         self.__sockfile = self.__sock.makefile(mode='r')
         return self.__negotiate_capabilities()
 
-    def cmd_obj(self, qmp_cmd):
+    def cmd_obj(self, qmp_cmd: QMPMessage) -> QMPMessage:
         """
         Send a QMP command to the QMP Monitor.
 
         @param qmp_cmd: QMP command to be sent as a Python dict
-        @return QMP response as a Python dict or None if the connection has
-                been closed
+        @return QMP response as a Python dict
         """
         self.logger.debug(">>> %s", qmp_cmd)
-        try:
-            self.__sock.sendall(json.dumps(qmp_cmd).encode('utf-8'))
-        except OSError as err:
-            if err.errno == errno.EPIPE:
-                return None
-            raise err
+        self.__sock.sendall(json.dumps(qmp_cmd).encode('utf-8'))
         resp = self.__json_read()
+        if resp is None:
+            raise QMPConnectError("Unexpected empty reply from server")
         self.logger.debug("<<< %s", resp)
         return resp
 
@@ -233,9 +270,13 @@ class QEMUMonitorProtocol:
         Build and send a QMP command to the monitor, report errors if any
         """
         ret = self.cmd(cmd, kwds)
-        if "error" in ret:
-            raise Exception(ret['error']['desc'])
-        return ret['return']
+        if 'error' in ret:
+            raise QMPResponseError(ret)
+        if 'return' not in ret:
+            raise QMPProtocolError(
+                "'return' key not found in QMP response '{}'".format(str(ret))
+            )
+        return cast(QMPReturnValue, ret['return'])
 
     def pull_event(self, wait=False):
         """